mebiuspower
CGN Ultra frequent flyer
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2 ARs - 2 different results
- Thread starter pilon
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Not acceptable by any "precision" standard. I would have been on board if the link took me to the FN site and to their AR barrels which I believe are known to be fairly accurate but 1 moa from a bolt action marketed as a marksman rifle is not really something to brag about. My Factory Remington 700 will do 1 moa easily with it's non chrome lined bore.
220 was correct in his statement that a chrome lined barrel does not shoot better than a non chrome lined barrel.
If you take 2 identical barrels and chrome line one and not the other you will almost always get better groups from the non chromed barrel. The chrome lined barrel will last longer though, unless the non chromed barrel is nitrided which is more consistent (more accurate) and forms a harder surface than chrome so it will also typically last longer.
A chrome lined barrel can shoot very well and there are plenty of accurate chrome lined barrels out there but all things being equal chrome is not the way to go these days.
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The issue is that non match bullets need to be stabilized within a very narrow range of stability factor something like 1.2 - 1.7 while match bullet with stabilize well from 1.2 - 2.2.
A similar issue occurs we comparing a match barrel to a non-match barrel. (chroming a barrel is great for barrel life but generally bad for accuracy).
For example, while the average rifle won't dont' it, you can get 1.25 MOA 5 shot groups using 40gr ammo from a 1-8" barrel but both the ammo and the barrel must be of very high quality.
This is because while barrel twist is critical to properly stabilize a projectile, bullet "overstabilization" does not exist and the underlying accuracy issues are caused by bullet/ammo quality issue and barrel quality which cause the bullet to "spin out" because of alignment and symmetry issues.
My point, a heavy "super match" barrel will shot "well" pretty much any bullet while a regular low grade barrel has a much more unpredictable behavior.
(An example I have a 1-8" AR which shoots very well with 60gr to 77gr bullets while my 1-8" Tikka shot very well from 40gr to 55gr and extremely well from 60gr to 77gr)
A similar issue occurs we comparing a match barrel to a non-match barrel. (chroming a barrel is great for barrel life but generally bad for accuracy).
For example, while the average rifle won't dont' it, you can get 1.25 MOA 5 shot groups using 40gr ammo from a 1-8" barrel but both the ammo and the barrel must be of very high quality.
This is because while barrel twist is critical to properly stabilize a projectile, bullet "overstabilization" does not exist and the underlying accuracy issues are caused by bullet/ammo quality issue and barrel quality which cause the bullet to "spin out" because of alignment and symmetry issues.
My point, a heavy "super match" barrel will shot "well" pretty much any bullet while a regular low grade barrel has a much more unpredictable behavior.
(An example I have a 1-8" AR which shoots very well with 60gr to 77gr bullets while my 1-8" Tikka shot very well from 40gr to 55gr and extremely well from 60gr to 77gr)
This ^^^
A shorter (lighter) bullet with less inertia is harder to stabilize and it will become unstable at a relatively short distance (300 yards ish) where a heavy (long) bullet requires a faster spin to stabilize but will retain stability to a much longer distance. Add in cheap components with poor consistency in their construction and results are even more erratic.
Barrel quality and ammo quality are very important if accuracy is the goal, I own AR barrels that cost well over $500 to have made from a quality blank that will shoot most ammo well but when you load some quality projectiles and work up a load for them they will shoot some amazing groups. My other AR with a rack grade nitrided barrel simply shoots decently with most ammo but nothing to get excited about.
We're getting a little off track here but to OP my advice is to try some different ammo and maybe try the same load but with better 55gr projectiles. You don't have to buy match grade or heavy projectiles but something like a Hornady 55gr FMJ-BT should perform just fine without breaking the bank, I would test the ones you have at 25 yards on a large clean backstop so you can see if your bullets are coming apart as they leave the muzzle. I wouldn't expect them to but I've also never tried those projectiles in my rifles so it's a possibility.
Also, don't get locked in on thinking you need a boat tail projectile to shoot accurately. A flat base projectile will usually outperform a boat tail within 300 yards, I don't know why that is but the benefits of the reduced drag of a boat tail don't really show until past 300 yards.
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The army was slinging 55gr bullets 600 meters with ZERO stability issues back in the mid 90's when I served, so I'm not sure where you are getting the idea that 55gr's loose stability and start to tumble at 300 yard"ish" statement from.
My understanding is that shorter bullets stabilize easier (it is actually the length that matters, not the weight, it just so happens that lighter bullets are normally shorter), but typically have a worse BC, so lighter/shorter bullets slow down faster and they get pushed around by the wind more causing them to perform poorly at longer distances, but they do not become unstable and keyhole unless they are passing the trans sonic barrier (like all bullets potentially do). The ability of a bullet to resist being pushed off target (or to "buck the wind") is very different from becoming unstable and tumbling.
Of all the loads experienced by a bullet, the friction with the air from it's rotation is potentially one of the smallest ones. When a bullet has an RPM of 220,000 when it leaves the barrel, at 300 meters ( less then half a second later) it MIGHT have slowed to 219,950.
FWIW
a 1:7 barrel, shooting a 55gr bullets at 3000 FPS, that bullet is spinning at a rate of 308,571 rpm when it leaves the barrel
a 1:9 barrel, shooting a 55gr bullets at 3000 FPS, that bullet is spinning at a rate of 240,000 rpm when it leaves the barrel
COULD a bullet be spun at an RPM high enough that it would disintegrate? I'm sure it could, but I have no idea what that RPM is and I'm sure it varies from one bullet make/model to the next
I've noticed that after around 300 yards the 55gr projectile groups start to open up, I'm not saying that it starts to tumble at that distance only that a round that was shooting 2 moa out to 300 yards starts to lose stability and opens up to 3 or 4 moa (or more) as it gets out further where a heavier (longer) projectile will remain fully stabilized and continue on to make the 2 moa groups all the way out to impact on target.
I watched a buddy of mine with a bolt action (full custom build) with a 7 twist barrel throw an 80gr projectile out to 1100 yards with enough precision to hit empty BBQ tanks every shot but my 12 twist varmint rifle with a 36gr projectile that shoots better than 0.5 moa to 200 yards can't make a recognizable group at 600 yards.
I've had projectiles come apart as they leave the muzzle of a 7 twist 223. It was years ago but I think they were cheap 55gr FMJ's, kind of a funny sound as they come off the muzzle and just a weird mess on the target if you're shooting close in. It all depends on the construction of the projectile. I've shot 36gr Varmint Grenade's from my 8 twist ACR barrel and they were moving at 3600fps if I recall correctly. No issues with bullet coming apart but those projectiles are designed for it and somehow still explode violently on impact. That rotational velocity also aids in fragmentation on impact
For God's sake, please stop saying it looses stability. It doesnt mean what you think it means... A bullet being pushed around but he wind is not unstable. It's a 223, it's a light 223, it's just being pushed around by the environment. If you could shoot it in a warehouse, with no environmentals to worry about, it would group just fine.
Let's just agree to disagree, not worth arguing over. I've seen what I've seen and you've seen what you've seen and we've interpreted the results a little differently.
After much reflection, here are 2 analogies to understand stability and "overstabilization".
* Stability : thinking of a perfect throw in football, if the rotation rate is too low it will keep on tumbling
* "Overstabilization" : an imbalance tire which wobbles badly at high speed but behaves well at low/normal speed.
A normal bullet is like an slightly imbalanced tire and a normal barrel would be a very slightly bent axle.
A match bullet is a virtually perfectly balance tire and a match barrel is an almost perfectly straight axle.
* Stability : thinking of a perfect throw in football, if the rotation rate is too low it will keep on tumbling
* "Overstabilization" : an imbalance tire which wobbles badly at high speed but behaves well at low/normal speed.
A normal bullet is like an slightly imbalanced tire and a normal barrel would be a very slightly bent axle.
A match bullet is a virtually perfectly balance tire and a match barrel is an almost perfectly straight axle.
Years ago it was advised against using AE 52gr HP in 1/7 AR because they came apart. It was an ammo for varmint bolt guns. It was the time when AR became popular and people just fed whatever cheap ammo they could find.
Whatever cast bullets used probably are not sturdy enough to take the fast rotation impeded by 1/7, and they are probably not concentric enough to be accurate in the first place. BTW, there are some leeways to the exact barrel twist, while it said 1/7 on the stamping, it could anything from below 1/8 to close to 1/6.
Whatever cast bullets used probably are not sturdy enough to take the fast rotation impeded by 1/7, and they are probably not concentric enough to be accurate in the first place. BTW, there are some leeways to the exact barrel twist, while it said 1/7 on the stamping, it could anything from below 1/8 to close to 1/6.
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